1. Field of the Invention
The invention relates to fixed cutter drill bits for drilling boreholes into the earth for the exploration and recovery of minerals, particularly petroleum and natural gas.
2. Description of the Related Art
Bit whirl has been documented as a major cause of early failure and reduced performance of PDC bits. To date numerous methods have been developed with the aim of eliminating bit whirl. These methods include summing the resultant forces from each PDC cutter on a drill bit and balancing them to give a low net resultant force, known as an Out of Balance force. This was further developed by Warren et al in two related publications (“Bit Whirl, a New Theory of PDC bit Failure”—J. Ford Brett, Thomas F. Warren, and Suzanne M. Behr; Society of Petroleum Engineers (SPE) Paper No. 19571) and (“Development of a Whirl Resistant Bit”—Thomas M. Warren, J. Ford Brett and L. Allen Sinor; SPE Paper No. 19572) such that a high imbalance force typical for bits at the time could be directed at a large low friction pad. This effect is similar to that used in gun drilling. Bits using this technique are commonly known as “Anti-Whirl” bits.
Other ways of controlling bit whirl include various forms of cutting structures mounted upon the body of the drill bit. These were developed such that a larger ridge is produced in the rock between cutters. This technique is typically known as “Tracking” and is detailed by Weaver et al (“A new PDC cutting structure improves bit stabilization and extends application into harder rock types”—G. E. Weaver and R. I. Clayton. SPE Paper No. 25734), and Mensa-Wilmot et al (“Innovative cutting structure improves stability and penetration rate of PDC bits without sacrificing durability”—Graham Mensa-Wilmot, Tony Krepp, SPE Paper No. 39310).
These, and many other related methods have been shown to be effective to an extent, but the effects of bit whirl are still evident in fixed cutter drill bits, and are still a significant cause for poor bit performance. Nonetheless, the understanding of bit whirl has generally improved the performance of fixed cutter drill bits.